// https://github.com/koajs/compose/blob/master/index.js
function compose (middleware) {
    if (!Array.isArray(middleware)) throw new TypeError('Middleware stack must be an array!')
    for (const fn of middleware) {
      if (typeof fn !== 'function') throw new TypeError('Middleware must be composed of functions!')
    }
  
    /**
     * @param {Object} context
     * @return {Promise}
     * @api public
     */
  
    return function (context, next) {
      // last called middleware #
      let index = -1
      return dispatch(0)
      function dispatch (i) {
        if (i <= index) return Promise.reject(new Error('next() called multiple times'))
        index = i
        let fn = middleware[i]
        if (i === middleware.length) fn = next
        if (!fn) return Promise.resolve()
        try {
          return Promise.resolve(fn(context, dispatch.bind(null, i + 1)));
        } catch (err) {
          return Promise.reject(err)
        }
      }
    }
  }

//
/* async function a(ctx, next) {
    console.log(1)
    const hello = await Promise.resolve("hello node.js")
    console.log(hello)
    await next()
    console.log("a end")
}

async function b(ctx, next) {
    console.log(2)
    const hello = await Promise.resolve("hello node.js")
    console.log(hello)
    await next()
    console.log("b end")
}

compose([a, b])({}) */

// same as below
async function a_new(ctx) {
  console.log(1)
  const hello = await Promise.resolve("hello node.js")
  console.log(hello)
  await b_new(ctx)
  console.log("a end")
}

async function b_new(ctx) {
  console.log(2)
  const hello = await Promise.resolve("hello node.js")
  console.log(hello)
  console.log("b end")
}

a_new({})